Screw mechanism
The screw is a mechanism that converts rotational motion to linear motion, and a torque to a linear force. It is one of the six classical simple machines. The most common form consists of a cylindrical shaft with helical grooves or ridges called threads around the outside. The screw passes through a hole in another object or medium, with threads on the inside of the hole that mesh with the screw's threads. When the shaft of the screw is rotated relative to the stationary threads, the screw moves along its axis relative to the medium surrounding it; for example rotating a wood screw forces it into wood. In screw mechanisms, either the screw shaft can rotate through a threaded hole in a stationary object, or a threaded collar such as a nut can rotate around a stationary screw shaft. Geometrically, a screw can be viewed as a narrow inclined plane wrapped around a cylinder.
Like the other simple machines a screw can amplify force; a small rotational force on the shaft can exert a large axial force on a load. The smaller the pitch, the greater the mechanical advantage. Screws are widely used in threaded fasteners to hold objects together, and in devices such as screw tops for containers, vises, screw jacks and screw presses.
Other mechanisms that use the same principle, also called screws, do not necessarily have a shaft or threads. For example, a corkscrew is a helix-shaped rod with a sharp point, and an Archimedes' screw is a water pump that uses a rotating helical chamber to move water uphill. The common principle of all screws is that a rotating helix can cause linear motion.
History
The screw was one of the last of the simple machines to be invented. It first appeared in Mesopotamia during the Neo-Assyrian period BC, and then later appeared in Ancient Egypt and Ancient Greece.Records indicate that the water screw, or screw pump, was first used in Ancient Egypt, some time before the Greek philosopher Archimedes described the Archimedes screw water pump around 234 BC. Archimedes wrote the earliest theoretical study of the screw as a machine, and is considered to have introduced the screw in Ancient Greece. By the first century BC, the screw was used in the form of the screw press and the Archimedes' screw.
Greek philosophers defined the screw as one of the simple machines and could calculate its mechanical advantage. For example, Heron of Alexandria listed the screw as one of the five mechanisms that could "set a load in motion", defined it as an inclined plane wrapped around a cylinder, and described its fabrication and uses,
including describing a tap for cutting female screw threads.
Because their complicated helical shape had to be laboriously cut by hand, screws were only used as linkages in a few machines in the ancient world. Screw fasteners only began to be used in the 15th century in clocks, after screw-cutting lathes were developed. The screw was also apparently applied to drilling and moving materials around this time, when images of augers and drills began to appear in European paintings. The complete dynamic theory of simple machines, including the screw, was worked out by Italian scientist Galileo Galilei in 1600 in Le Meccaniche.
Lead and pitch
The fineness or coarseness of a screw's threads are defined by two closely related quantities:- The lead is defined as the axial distance the screw travels in one complete revolution of the shaft. The lead determines the mechanical advantage of the screw; the smaller the lead, the higher the mechanical advantage.
- The pitch is defined as the axial distance between the crests of adjacent threads.
Handedness
The helix of a screw's thread can twist in two possible directions, which is known as handedness. Most screw threads are oriented so that when seen from above, the screw shaft moves away from the viewer when turned in a clockwise direction. This is known as a right-handed thread, because it follows the right hand grip rule: when the fingers of the right hand are curled around the shaft in the direction of rotation, the thumb will point in the direction of motion of the shaft. Threads oriented in the opposite direction are known as left-handed.By common convention, right-handedness is the default handedness for screw threads. Therefore, most threaded parts and fasteners have right-handed threads. One explanation for why right-handed threads became standard is that for a right-handed person, tightening a right-handed screw with a screwdriver is easier than tightening a left-handed screw, because it uses the stronger supinator muscle of the arm rather than the weaker pronator muscle. Since most people are right-handed, right-handed threads became standard on threaded fasteners.
Screw linkages in machines are exceptions; they can be right- or left-handed depending on which is more applicable. Left-handed screw threads are also used in some other applications:
- Where the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten due to fretting induced precession. Examples include:
- *The left hand pedal on a bicycle.
- *The left-hand screw holding a circular saw blade or a bench grinder wheel on.
- In some devices that have threads on either end, like turnbuckles and removable pipe segments. These parts have one right-handed and one left-handed thread, so that turning the piece tightens or loosens both threads at the same time.
- In some gas supply connections to prevent dangerous misconnections. For example, in gas welding the flammable gas supply line is attached with left-handed threads, so it will not be accidentally switched with the oxygen supply, which uses right-handed threads.
- To make them useless to the public, left-handed light bulbs are used in some railway and subway stations.
- Coffin lids are said to have been traditionally held on with left-handed screws.
Screw threads
Thread angle
The thread angle is the included angle, measured at a section parallel to the axis, between the two bearing faces of the thread. The angle between the axial load force and the normal to the bearing surface is approximately equal to half the thread angle, so the thread angle has a great effect on the friction and efficiency of a screw, as well as the wear rate and the strength. The greater the thread angle, the greater the angle between the load vector and the surface normal, so the larger the normal force between the threads required to support a given load. Therefore, increasing the thread angle increases the friction and wear of a screw.The outward facing angled thread bearing surface, when acted on by the load force, also applies a radial force to the nut, causing tensile stress. This radial bursting force increases with increasing thread angle. If the tensile strength of the nut material is insufficient, an excessive load on a nut with a large thread angle can split the nut.
The thread angle also has an effect on the strength of the threads; threads with a large angle have a wide root compared with their size and are stronger.
Image:Screw thread forms.png|thumb|upright=2.7|center|Standard types of screw threads: V, American National, British Standard, Square, Acme, Buttress, Knuckle
Types of threads
In threaded fasteners, large amounts of friction are acceptable and usually wanted, to prevent the fastener from unscrewing. So threads used in fasteners usually have a large 60° thread angle:- V thread - These are used in self-tapping screws such as wood screws and sheet metal screws which require a sharp edge to cut a hole, and where additional friction is needed to make sure the screw remains motionless, such as in setscrews and adjustment screws, and where the joint must be fluid tight as in threaded pipe joints.
- American National - This has been replaced by the almost identical Unified Thread Standard. It has the same 60° thread angle as the V thread but is stronger because of the flat root. Used in bolts, nuts, and a wide variety of fasteners.
- Metric thread - These threads are specified and common for ISO and DIN standards.
- Whitworth or British Standard - Very similar British standard replaced by the Unified Thread Standard.
- Square thread - This is the strongest and lowest friction thread, with a 0° thread angle, and does not apply bursting force to the nut. However it is difficult to fabricate, requiring a single point cutting tool due to the need to undercut the edges. It is used in high-load applications such as jackscrews and lead screws but has been mostly replaced by the Acme thread. A modified square thread with a small 5° thread angle is sometimes used instead, which is cheaper to manufacture.
- Acme thread - With its 28° thread angle this has higher friction than the square thread, but is easier to manufacture and can be used with a split nut to adjust for wear. It is widely used in vises, C-clamps, valves, scissor jacks and lead screws in machines like lathes.
- Buttress thread - This is used in high-load applications in which the load force is applied in only one direction, such as screw jacks. With a 0° angle of the bearing surface it is as efficient as the square thread but stronger and easier to manufacture.
- Knuckle thread - Similar to a square thread in which the corners have been rounded to protect them from damage, also giving it higher friction. In low-strength applications it can be manufactured cheaply from sheet stock by rolling. It is used in light bulbs and sockets.